It depends on what you wanna do
Well, that might answer your question. As far as I know, no "mouse" exists on watch but that doesn't mean you can't control your PC with your watch.
You might use Unified Remote, tasker and it's smartwatch's version. Unified remote is an app that enables you to control your PC wirelessly from your phone (for example, you can play or pause windows media center, launch any program...), very useful and customizable with paid version. Then, tasker is a really really useful app (maybe a bit hard to understand at first) with which you'll be able to activate only few options of your apps. Tasker live extension shows these options on the watch. I've never tried myself but give it a try and let us know.
I don't know if it's clear enough but, for example, I use sonos system (those are speakers connected to wifi that you control with a special app on your phone), using tasker and tasker extension, I can control sonos (play, pause, skip, volume,...) using my watch.
Of course, as the guy above me said, this will never as easy and fast as on your phone. However there are apps that are intended for controlling your PC.
PS : excuse me for my not-that-good-english.